It is proposed to continue the investigations of satellite bacteriophage P4 and of its helper, bacteriophage P2, and in particular the analysis of the interactions between P4 and P2. Planned are: (1) further studies of the derepression of, and phage production by lysogenic cells, resulting from their infection with a heteroimmune phage, as observed in the P2 - P4 system; (2) related studies concerning phage production by cells containing both P2 and P4 as prophages; (3) further analysis of the mechanism(s) employed by P4 to transactivate helper genomes, especially by using P2 mutants and relatives of P2 that cannot be transactivated by P4; (4) further studies of the bacterial gro mutation, which blocks growth of P2, and of the nature of the ogr mutation of P2, which enables P2 to overcome the gro block, and studies of the effects of gro and ogr on P2 - P4 interactions; (5) isolation and characterization of P4 mutants, especially with regard to their use for (1) - (4).